Conventionally, an air flow rate measurement device is proposed (for example, refer to Patent Literature 1), which measures a humidity of air in addition to a flow rate of air drawn into an engine of a vehicle.
The air flow rate measurement device is installed by being inserted into an intake passage from an insertion hole of an intake pipe. The air flow rate measurement device includes a case forming a bypass channel which takes in a part of air flowing through the intake passage, a flow rate sensor installed in the case, and a humidity sensor installed outside of the case.
Meanwhile, when heat of the engine gets across to the case, a sensing part of the humidity sensor may receive radiant heat from the outer wall of the case. Especially, in case where the sensor is an electric capacitance type humidity sensor, if the temperature rises, the amount of moisture existing in air becomes different while the relative humidity is the same. For this reason, the accuracy for measuring the humidity may be lowered by the radiant heat from the outer wall of the case.
Then, in the air flow rate measurement device of Patent Literature 1, the humidity sensor is installed so that the sensing part is located to face an opposite side opposite from the outer wall of the case so as to reduce the influence of the radiant heat from the outer wall of the case.
However, when the sensing part of the humidity sensor is attached to the air flow rate measurement device to face outward of the case, the sensing part may be damaged. Specifically, the surface of the sensing part may contact the inner wall of the insertion hole, when the air flow rate measurement device is inserted into the intake passage to pass through the insertion hole from the outer side of the intake pipe.
Then, in the air flow rate measurement device of Patent Literature 1, a protection component which protects the surface of the sensing part is installed on the outer side of the humidity sensor. The case has a fitting part fitted with the inner wall of the insertion hole of the intake pipe, and a main body extending from the lower end of the fitting part into the intake passage. The protection component is provided to project from the lower end of the fitting part in a direction parallel to the outer surface of the main body.
However, since the protection component is provided to project outward from the case in the conventional air flow rate measurement device, the protection component becomes a resistance for the air passing outside of the case, such that the pressure loss of the air flowing through the intake passage increases.
Moreover, since a space for installing the protection component is needed on the lower side of the fitting part, the size of the fitting part in the width direction becomes large on the outer side of the case in the width direction. For this reason, the size of the case increases.